All modern ships are equipped with a device able to reduce the drag coefficient, called trim wedge. Such a device is applied in the stern area below the transom. Such a device consists of a wedge-shaped appendix that extends across the width of the ship and is substantially defined by a flat vertical surface facing towards stern as a continuation of the transom and by an inclined surface facing towards the bow. When the ship proceeds in the forward direction, the inclined surface is hit by water. A very high lift force is thus generated which changes the trim of the ship, significantly improving the propulsive performance thereof. In order to be effective, a trim wedge should be placed with its lower portion below the waterline.
The trim wedge cannot be used on ships intended to operate in icy sea areas. In navigation in the ice, in the case of reverse motion the vertical flat surface of the trim wedge would impact the ice. The impact area would be very large, given the extension of the trim wedge. While the ship could have a powerful propulsion system, the specific pressure would be very low, and therefore insufficient to cause the fracture of the ice. In addition, the trim wedge would cause a gradual accumulation of ice in the stern area resulting in blockage of the ship in the reverse motion.
Due to these constraints, the ships used for navigation in icy waters, even if only for some periods of the year, are never equipped with stern trim wedge. In particular, the adoption of the trim wedge would make manoeuvres in icy port areas considerably difficult, if not impossible, the reverse motion being in fact prevented.
Therefore, in the ships intended for navigation in icy waters, the need to ensure adequate manoeuvrability has led to sacrifice the propulsive performance in open water, giving up the trim wedge.
A need deeply felt in the field of ships used for navigation in icy waters therefore is to improve the propulsive performance in open water, without renouncing to an adequate manoeuvrability in icy waters.